APRS.TXT AUTOMATIC PACKET/POSITION REPORTING SYSTEM (APRS)
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Document version: 8.5.3 (10 year anniversary update)
Document dated: 18 Sept 2002 (Previous version was 1 Mar 2000)
Author(s): Bob Bruninga, WB4APR@amsat.org
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Home page: www.aprs.org or www.ew.usna.edu/~bruninga/aprs.html
APRS is a real-time tactical digital communicatons protocol for exchanging
information between a large number of stations covering a large (local)
area. As a multi-user data network, it is quite different from
conventional packet radio.
APRS is different from regular packet in four ways. First by the
integration of maps and other data displays to organize and display
data, second, by using a one-to-many protocol to update everyone in real
time, third, by using generic digipeating so that prior knowledge of the
network is not required, AND FORTH, since 1997, a worldwide transparent
internet backbone, linking everyone worldwide. APRS turns packet radio
into a real-time tactical communications and display system for emergencies
and public service applications (and global communications). Normal
packet radio is useful in passing bulk message traffic (Email) from point-
to-point, but it does not do well at real time events where information
has a very short life time and needs to get to everyone quickly.
APRS is a LOCAL RF network. Although the Internet monitors APRS worldwide,
this is not the primary objective. But like all of our other radios, how
we use APRS in an emergency of special event is what drives the design of
the APRS protocol. Although APRS is used 99% of the time over great
distances, and benign conditions, the protocol is designed to be optimized
for short distance real-time crisis operations on RF.
APRS provides universal connectivity to all stations in the net by avoiding
the complexity and limitations of a connected network. It permits
any number of stations to exchange data just like voice users would on a
voice net. Any station that has information to contribute simply sends
it, and all stations receive it and log it. Secondly, APRS recognizes
that one of the greatest real-time needs at any special event or emergency
is the tracking of key assets. Where is the Event Leader? Where are the
emergency vehicles? Whats the Weather at various points in the County?
To answer these questions, APRS transmits and captures the location and
status of all stations. It can be used over any 2-way radio system
including HAM, CB, Marine Band, etc. See CB.txt, Boats.txt, Aircraft.txt.
APRS is on 144.39 throughout the North American Continent. Other
countries may use other frequencies. Check locally.
Included on the original APRSdos distribution disk (and this web site), are
several README text files on applications of APRS such as for weather nets,
direction finding, plotting satellite contacts, and monitoring DX clusters.
You must read at least the README.1st, DEMOS.txt, NewNotes.txt and HELP.txt
files. APRS uses UI frame broadcasts and map displays. There are three
major display subsystems and a number of other minor displays as follows:
LIST-STATUS - This display maintains a list of the latest UI status
packet from each station. In effect, this is a multi-station one-line
broadcast status system. On a DX cluster freq, this display accumulates a
list of all users and what was their latest command to the cluster.
POSITIONS - This display maintains a separate list of the positions of
each station often including a brief comment. They can also contain DF
and or WX info.
MAPS - Maps from 300 yards up to 4000 miles can be displayed. All
other APRS stations will be plotted. Stations reporting a course and
speed are dead-reckoned to their present position. Overlay databases of
the locations of all APRS DIGIpeaters, National WX Service sites and
even HAM stores are built in. You can zoom in to any point on the globe!
MESSAGES - In addition to the one-line STATUS packets, two-way
messages with acknowledgment are supported. All incoming messages alert
the user on arrival and are held on the MESSAGE screen until Killed.
BULLETINS - This screen maintains a list of active BULLETINS. These
are important multi-line BULLETINS addressed to everyone.
TRAFFIC - Shows the last 42 messages monitored between other stations
on the net. The MAPS-PLOTS-TRAFFIC command will connect these stations
with lines on the map.
LIST-LOG - This display is a time sequenced log of every new
status or message received. This is in contrast to the LATEST display
which only shows the most recent status packet per station.
HEARD LOG - Displays the total number of packets per station per hour.
These statistics show the connectivity of the network over varying paths,
such as HF, or to see when stations enter and leave the net.
DIGIPEATER LIST - This display displays the raw packet header so that
APRS users can see what digipeater paths are being used by other stations.
The proper use of digipeaters is important in an APRS network. An asterix
in the first column means that you hear the other station direct without a
digipeater! Change your UNPROTO path to NONE to chat with those stations.
ANALYZE - This display is a great tool for monitoring the APRS local
network and giving selected displays on numerous statistics abotu the local
RF network. Just digipeaters, Just mobiles, Just WX, selection by software
version, object type, or a display of the number of hops used by each
station and digi.
STATION TRACKING. Although APRS automatically tracks mobile GPS/LORAN
equipped stations, it also tracks perfectly well with manual reports or
GridSquares. APRS will use a GridSquare in brackets at the beginning of
any packet. Additionally, any station can place an object on his
map including himself and within seconds that object appears on all other
station displays. In the example of a parade, as each checkpoint with
packet comes on line, its position is instantly displayed to all in the
net. Whenever a station moves, he just updates his position on his map and
that movement is transmitted to all other stations. To track other event
assets, only one packet operator needs to monitor voice traffic to hear
where things are. As he maintains the positions and movements of all
assets on his screen, all other displays running APRS software display
the same displays. There is also a Tracking command on the P display that
will cause APRS to keep the map display always centered on a selected
object.
GRID SQUARES: Because of the ambiguity of a grid-square position report,
APRS will not display a 4 or 6 digit gridsquare report on map ranges
less than 128 and 8 miles respectively. Stations reported in the same
grid square are randomly offset to avoid cluttering of callsigns on top of
each other. Grid Squares are brief; six characters vice seventeen. APRS
uses this to advantage in GRID-SQUARE mode, for Meteor Scatter, SAREX,
or the PACSATS.
ACARS: If your TNC has an ACARS modem for receiving Airline digital packets
on 131.55 MHz, then the optional APRSair.EXE will plot the positions of all
aircraft in range (usually up to about 200 miles out).. See ACARS.txt
USING APRS ON ALL DIGITAL CHANNELS: Use APRS posit packets on all
frequencies as a general purpose network topology display on ANY packet
frequency! See the PROTOCOL.txt for details on APRS formats and see
FRQCOORD.txt for suggestions for using APRS as a freq-coord display tool.
SATELLITES: Use the special version APRStk.exe to enjoy APRS and also
see all the satellites when they come in view. At least 5 satellties
permit APRS digipeating and 7 can be received by your Kenwood Radios
and auto-tuned by APRStk for each pass! APRS is ideal for the short but
congested satellite pass. APRS only requires one successful packet for
everyone to see each successful station compared to the typical *CONNECTED*
SAREX mode which requires 5 successful packets. Thus APRS reduces channel
loading, while capitalizing on the most fascinating aspect of the amateur
radio hobby, and that is the display on a map of the location of those
stations. See SPACE.txt.
FOX HUNTING OR DIRECTION FINDING: APRS is an excellent tool for plotting
the location of a hidden transmitter, balloon, or interfering signal.
APRS has several powerful DF tools:
1) Plots the positions and Bearing lines of all participating stations
whether mobile or fixed
2) Plots the overlaping signal strength contrours for OMNI-DF reports.
This techinque even plots big BLACK circles for NULL reports so
that you see all the areas where the FOX is NOT! This OMNI technique
is very powerful and locate a jammer to a neighborhood with NO
beams or special equipment.
3) Fade-Circle Search and Rescue technique for single station signal
strength location using only a OMNI antenna
4) Optional automatic DF interface to Doppler DF units for automatic
plotting of DF bearings.
For more DF info, see the DF.txt file or the "dfing.html" on this site.
WEATHER STATION REPORTING: APRS position reports can also include the wind
speed and direction, as well as other important weather conditions. APRS
supports a serial interface option to the ULTIMETER and DAVIS home weather
stations to do this automatically. All weather stations show up as a blue
circle, with a line indicating wind speed and direction. The APRS NEXT
command will successively highlight each WX station in turn, for WX at a
glance. APRS also has a database of the locations of most NWS sites and
can crunch a file of NWS hourly WX conditions for display. Finally, APRS
users can set WX alarms and be alerted when WX conditions exceed those
values. See WX.txt.
DX CLUSTERS: APRS an ideal tool for the DX cluster user. Not only does
he get to see all DX spots on the map, but by operating in the monitor only
mode, he has reduced the overall packet load on the DX cluster. This is
a benefit to everyone on the channel. Also the APRS monitoring station
will see the SPOT as soon as the first station gets it, rather than later
on down the list. See DXclustr.txt.
INTERNET: APRSdos no longer supports the APRS-Internet access, since it
is now just too big. And APRS is a local RF network anyway for most
local practical applications.
Other APRS clients will attempt to display the 30,000 or os other APRS
stations worldwide, though this can be flitered as needed to cut the
clutter. The real beauty of APRS-Internet system is that is gives long
distance connectivity automatically to APRS MESSAGES. All packets
everywhere go IN to the APRS-IS, and if needed, any station-to-station
messages go out the other end on RF to another user.
FREQUENCY COORDINATION: Every packet asset on every frequency should
include a position or at least gridsquare in all routine BEACONS. This
alows APRS to be used to monitor network topography on any frequency.
Thus, APRS makes an excellent tool for frequency coordination. In fact,
javAPRS is now used by TAPR to maintain the entire digital data base for
North America.
PROTOCOL - Although APRS redundantly transmits data, a fundamental precept
is that old data is less important than new data. All APRS packets are
repeated at an ever decreasing rate. Each new packet is transmitted
immediately, then 10 seconds later. After every transmission, the period
is doubled. After 20 minutes only six packets have been transmitted. From
then on the rate remains at 10 minutes for local 1-hop nets and every 30
minutes for stations using 2 or more hops. All transmissions can be toggled
off using the CONTROLS-XMT command, or forced at any time with the X key.
See PROTOCOL.txt
COMMANDS: The keyboard is always active. Almost every key does something
or brings up additional menus. (APRS processing of packets on the air is
continuous EXCEPT while waiting for the user response to a prompt. These
prompts are surrounded with a yellow). Commands fall into 3 categories:
APRSdos SCREENS:
Space Key - Display map and all station locations
L - LIST - Menu for DX, LOG, TELEMETRY, RESOURCES screens
P - Positions - Displays a list of all stations reporting positions
B - BULLETINS - Keeps a list of all BULLETINS heard
R - Read Messages - Displays all of your in and out messages
T - Traffic - Displays the last 23 lines of messages between stns
D - Digis Used - Displays the digipeater paths being used by others
H - Heard Log - Displays packets per hour per station for 24 hours
V - VIEW - Displays all packets on a scrolling screen
SUB-MENUS:
F1- Help - Select from a MENU of HELP commands
C - Controls - Display a one line control panel
F - FILES Menu - For Loading/Saving files, or Replaying tracks
I - Input commands - Used to input posits, DF info or add OBJects to map
O - OPERATIONS - Several commands for normal operations
M - MAP Functions - Functions dealing with maps
W - Weather Menu - Displays the number of beacons per hour per station
MESSAGES:
R - READ - Displays your incomming and outgoing messages
S - Send - Sends traffic to a station
E - Erase - Erases outgoing traffic lines
K - Kill - Kills incomming traffic lines
T - Traffic - Displays message traffic between other stations
DEMONSTRATION FILE: To see how APRS looks in our area, FILES-LOAD the file
named WASHDC.BK. To see the tracking of the GPS equiped Army/Navy game
football run, FILES-LOAD FBALL.BK and do a FILES-REPLAY of the file named
FBALL93.HST. To see the Marine Corps marathon, load MARATHON.BK and replay
the MARTHON.HST file. See Details in README.1st.
SELECTING STATIONS: The cursor can be moved to select any station on the map
using the arrow keys. When near any station symbol, the ENTER key will
"select" the station. Detail information on that station will be displayed
on the bottom of the screen. Alternatively, use the + and - keys to step
through each station one by one. or the <> to step through objects on the
current map scale only. You may also use the cursor on the P or L-list to
select a station or object. Once sselected, several functions may be
performed:
ALL - Displays a list of all beacons from that station.
EDIT - Change the position packet text
MOVE - Move the station by moving the cursor
DELETE - Delete the station.
FOLLOW - Force this object to FOLLOW a course (APRSdr only)
UPLINK - Toggle on/off transmiting the object to others
ALARM - Sets an alarm if that station ever moves.
TRACK - Always center display on selected station as it moves
SPECIAL- Mark stations for selective display with the JUST command
The select function also works on the LATEST and POSITION display lists by
using the up/down arrow keys.
REPLAY: Replay past track histories either from memory or from a file.
Tracks are kept in on-line memory until 150 have been saved, and then are
saved to a HISTORY file. During REPLAY, use the following commands:
C - CALLsigns on/off
HOME - Homes map to presently displayed station
SPACE- Redraws the present map to remove track clutter
F - Faster. Speeds up playback
G - Overlays the Civil Air Patrol Search and Rescue grids
M - Medium replay speed
P - Pause
S - Slow. Slows down playback
Q - Quit playback.
PgUp/PgDn - Zoom in and out
FILES: All APRS files are retained in five different sub-directories of
BAKS, LOGS, HSTS, SYSTEM, MAPLISTS and README. There are several
other files used by the system:
MAPLIST.USA - The file that lists all map files to be used by APRS.
These MAPLISTS are all in the MAPLISTS directory. Change
to different MAPLISTS using the MAPS-CHANGE command.
BACKUP .BK - Automatic backup of system every time program is quit. It
can be loaded quickly using the FILES-LOAD-B command.
Or Automatically by invoking APRSXX/B at the DOS prompt.
MAPFIX40.EXE - A powerful program for fixing, drawing, and modifying maps!
SYSTEM (DIR) - Contains the following required files
IintTAPR.TNC - Setup parameters for your TNC (InitAEA.TNC for AEA)
RESTORE.TNC - TNC commands used to restore your TNC after quitting APRS.
CAPGRIDS.DAT - A file of the CAP Sectional Aeronautical charts
DXCALLS.DAT - Callsign prefix-to-LAT/LONG database for DX spots
XXXX.SYS - Numerous brief files for APRS internal screens
OTHER FILES:
METAR.TXT - A sample file used to load National Weather Service data
NWSPOSNS.POS - A file of the locations of all NWS sites
CHESSBOARD: To demonstrate the flexibility of APRS in reporting the
movement of objects in a net, there is a 2 mile chessboard in the center
of the Gulf of Mexico. Two APRS stations can place pieces on the board
with the INPUT-ADD command or can move existing pieces using the Cursor,
SELECT and INSert commands! An easy way to begin a game, is to load the
CHESS.BK file which already has all 32 pieces saved. By selecting the
alt-SETUP-OTHER-GAME command APRS not only keeps track of the move numbers
but also changes the packet address from APRS to GAME, so that others on
frequency will not see their PLists cluttered with the pieces as they are
moved. Also, GAME mode only UPLINKS the LATEST move, to minimize QRM.
You may consider going to an unused frequency so as not to clutter an
active APRS net, however.
If you want to Kibitz a game in progress, also load the CHESS.BK file
and use the CONTROLS-FILTERS-OTHER command so that your APRS will monitor
OTHER packets on the channel so you will see the GAME packets. If you
missed the beginning of the game, ask one of the players to use his P
list to SELECT and UPLINK any piece locations you missed.
OTHER SPECIALIZED VERSIONS OF APRSdos:
APRSmax.exe This program removes some rare features from the full up
APRSdos to make more room for more stations. This version will capture
and display over 250 stations compared to the original 100. When either
system is full, the oldest station is deleted to make room for each new
station heard.
APRSdr.EXE: For tracking known OBJECTS that move at KNOWN speeds over
a KNOWN course, such as the runners in a Marathon, use APRSdr. This
program will Dead Reckon objects along the course, so that you can
reasonably track the progress of the event, without having to strap
a GPS/TNC and RADIO to the head of the lead runner. All GPS mobiles
will also be tracked. This is a trememdous visual aid for NET CONTROL.
APRSwx.exe: This version minimizes serial port overflow if you are
running an interface to an Ultimeter 2000 WX station.
APRSmin.exe: This version runs in less space, about 450K instead of 550.
APRSair.exe: For tracking commercial aircraft using the ACARS system.
APRSnet.exe: For tracking all packets on the INTERNET www.aprs.net:10151
APRStk.exe: Includes Satellite tracking and Kenwood radio tuning
APRSdata.exe: A local tiny-web-page server for all mobile kenwoods